I had to include Engine.h to my Actor header file.
I also had to change the FString::FromInt( ) calls to, FText::AsNumber( ) and TEXT( ) to, FText::AsCultureInvariant( ) for UpdateTimerDisplay( ) and CountdownHasFinished( ), in that order.
When you create a project, it should create a header file with your project name “ProjectName.h”, which includes all the base Header Files (including Engine.h) for the engine to work properly. You should only need to include your ProjectName.h file into your classes (which is done automatically with the Create a Class Wizard in the editor).